The Molecular Organic Geochemical Study Of Paleo-To Neoproterozoic (1.8-0.85GA) Sediments From Jixian Strata Section, North China

The Precambrian, one of the most important stages in the history of the Earth, so far, is still less known to us mostly because of the absence of intact morphological fossils in the Precambrian strata. This study systemically investigated the molecular (including soluble saturates and hydrocarbon products of keogen degradation) and carbon isotopic (including soluble hydrocarbons, kerogens and coexisted carbonates etc.) characteristics of Paleo- to Neoproterozoic (1.8-0.85Ga) sediments from the famous Jixian strata section, North China. The environmental and biological evolutions of Proterozoic Yanshan Basin companied by the global evolution were also discussed in detail with these molecular and isotopic data combined with independent palaeoenvironmental and palaeontologic information as acquired from these strata.Analogous hydrocarbon types were detected in almost all samples from Jixian strata section. It is worth noting that the old steranes and the so far oldest gammacerane was detected in the l.SGa shales from Chuanlinggou Formation, which denote the presences of eucaryotes and protozoans respectively. The agreement of molecular characteristics with palaeoenvironmental and fossils data as derived from these strata suggests that the biomarkers preserved in these Proterozoic sediments are good indicators for the evolution of the palaeoenvironment and the early life in Yanshan basin.The stable carbon isotopic characteristics of kerogen (813Ct) and coexisting carbonate (Sl3Ccait>) derived from the strata show a rapid decline of atmospheric CO2 concentration before 1.55Ga, and a stable concentration after 1.55Ga. It is confirmed that the difference (Ac) between the isotopic compositions of carbonate and associated kerogen can reflect the extent of carbon isotope fractionations accompanying biological usage of COa- The cyclic variations along the strata sequence exhibited by AC record may reflect the periodical marine transgression and regression that occurred in Yanshan basin during the Proterozoic age, and in turn, the changes of biocommumties adaptable to the changed environments.The carbon isotopic relationships among kerqgens and n-alkanes and isoprenoids from these strata are consistent with the hypothesis that before 1.4Ga both n-alkanes and isoprenoids were mainly derived from heterotrophic halophiles and other possible archaebacteria, but after 1.4Ga only isoprenoids were still mainly originated from the archaebacteria while n-alkanes were significantly derived from primitive organic matter of photosynthetic organisms such as cyanobacteria. The change of biological origins of n-alkanes around 1.4Ga may suggest a marked decline of heterotrophic reworking to primitive lipids after 1.4Ga. This decline might be related to the formation of stable epicontinental sea characterized by shallow water environment. The isotopic relationship between individual hydrocarbons and associated kerogens derived from Jixian strata section may suggest special biogeochemical processes existed in closed Proterozoic Yanshan Basin compared to those of open sea system.Rock-Eval analysis and kerogen degradations show that most of the kerogens from the strata section are over mature. Only the kerogen acquired from Hongshuizhuang Formation can meet the chemolysis and thermolysis of kerogen. Abundant structural and biological original information was obtained from this kerogen based on the integrated applications of multiple organic geochemical methods significantly including RTCO and Py-GC-MS. Moreover, the indigenous natures of soluble hydrocarbons derived from Hongshuizhuang Fm. are also suggested by these integrated applications.The molecular and other geochcmicaf correlations between ttongshuizhuang i^m. and Nonesuch Fm. from Central North American Rift System show the possible linkage between North China Block and Laurentia Block before 1.1-1.2Ga.Features and innovations in this study: (1) Longest successive time scale of Precambrian samples is studied; (2) The environmental and biological significances of the d...